1. Field of the Invention
This invention relates to a wear-resistant protective film for a thermal head and a method of producing a wear-resistant protective film for a thermal head.
2. Prior Art
Thermal heads are extensively used as printing heads for computers, word processors, facsimile machines, etc. The head has a number of dots or resistance heating elements of polysilicon or the like arranged in a matrix and which are selectively supplied with a current to print characters by heat transfer through a printing ribbon onto paper. Since the paper is moved in sliding contact with the thermal head surface, the resistance heating elements must be protected on the surface with a highly wear-resistant protective film.
Each spotlike printing element of the thermal head, as shown in FIG. 1, comprises, from the base upward, a substrate 1 of alumina or the like, a regenerative layer 2 of glaze glass or the like, a heating-element layer 3 of polysilicon or the like, electrodes 4, 5, and a wear-resistant protective film 6. In the figure the numeral 7 designates a heat-developing zone.
The protective film 6 generally is required to have high hardness, limited internal stresses attributable to heat, composition and structure, resistance to wear, and stability to moisture, alkalis, acids and the like. Various materials have hitherto been studied, including such known materials of Si—O—N, Si—Ti—O—N, Si—La—O—N, Si—Al—O—N systems.
Wear-resistant protective films conventionally formed by sputtering crack frequently. Once cracked, such a film allows moisture in the atmosphere to gain entrance through the crack into the thermal head to corrode it, often leading to film separation. Among the factors responsible for the cracking are the development by dint of a peening effect of the internal stresses due to heat, composition, and structure, and the lack of toughness. A particularly serious factor is inadequate step coverage of steplike portions. Ideally, the wear-resistant protective film is formed as shown in FIG. 1. In the actual film-forming process the film material fails to cover the steps fully, as at 8, 8 in FIG. 2, giving cause for cracking as early as the formation of the film. Intrusion of water or repeated exposure to heat would invite premature cracking at the steps.
This step coverage problem can be overcome by the use of a biased radio frequency (RF) sputtering technique in forming a wear-resistant protective film (Japanese Patent Application Public Disclosure No. 135261/1988). The biased RF sputtering proves excellent in covering steps, but the attendant peening effect and incorporation of sputter gases (Ar, Kr, etc.) into the protective film increase the internal stresses. Consequently, the film cracks easily and becomes less adherent.
Although the above reference describes that cracks and peeling are avoided, the reality is that cracks are prone to develop due to the internal stress, according to the inventors tests. Moreover, there is no disclosure in the reference on forming two or more layers while varying the bias for sputtering.
The Problem to be Solved by the Invention
As stated above, the conventional wear-resistant protective film is prone to crack or corrode owing to poor step coverage by sputtering. Biased RF sputtering too tends to cause cracking due to increased internal stresses and low adherence.
Means for Solving the Problem
Therefore, the present invention aims at providing a wear-resistant protective film for a thermal head and a method of producing a wear-resistant protective film which has little possibility of cracking ascribable to internal stresses or step coverage.
The present invention resides in a method for producing a a wear-resistant protective film for a thermal head, which comprises sputtering a wear-resistant protective film on a thermal head which includes a substrate, and a heat-developing layer and a pair of electrodes formed on either the substrate or a heat-regenerative layer formed thereon, characterized in that a part of the wear-resistant protective film is formed under a larger bias and another part under no or a smaller bias. The present invention also resides in the wear-resistant film thusly formed. The bias may be a DC bias or an AC bias for an electrically conductive protective film and an AC bias is used for an electrically insulating protective film, usually, a high frequency bias is preferred.
According to the invention, a layer of good step coverage formed by sputtering under a larger bias (preferably RF) in one part of the wear-resistant protective film prevents the intrusion of water that can cause corrosion and cracking. Also, a layer of low internal stress is formed under no bias or a smaller bias, adjacent to the layer sputtered under the larger bias, the internal stress level throughout the film is reduced. This inhibits development of cracks with the internal stresses produced by sputtering under the larger bias. These factors combine to prevent cracking which otherwise results from the ingress of moisture or internal stresses.
Sputtering with a larger bias is defined as a sputtering (preferably, RF sputtering) under a bias in the range of −50V and −200V, more preferably −60 and −120V. Sputtering with no bias or a smaller bias is defined as a sputtering under zero bias or a bias less than two third, more preferably from one half to one tenth, of the larger bias. If the protective film is electrically conductive, AC or DC voltage bias may be used. If the protective film is an insulator a AC voltage bias is usually used because an AC voltage bias is used for protective film of any electrical properties.
According to the present invention a superior wear-resistant protective film for thermal heads is produced which comprises a material selected from metal oxides, metal nitrides, or mixtures thereof, such as Si—O—N, Si—Ti—O—N, Si—La—O—N, Si—Al—O—N, Si—Sr—O—N, Si—Mg—O—N or mixtures of these materials, having a concentration of sputtering gas varying in the direction of thickness of the protective film. The metals here mean that ordinary metals such as Ti, Al and the like, B in the Group IIIa and C, Ge and Si in Group IVa, preferably Si.
The layer or layers formed with no bias or a smaller bias contains the sputtering gas such as Ar or Kr in an amount of 0–3 at % and develops little internal stress and accordingly no crack is observed.
The layer or layers formed with a larger bias contains the sputtering gas in an amount of 2–10 at % (but more than the layer or layers formed with no or smaller bias) and exhibits a good step coverage.
The thickness of the film deposited by the larger bias desirably ranges between 0.1 μm and 5 μm, more desirably between 0.5 μm and 3 μm. If the film is thinner than 0.1 μm the step coverage is inadequate, allowing the ingress of moisture. If it is thicker than 5 μm the internal stresses increase to excess.
On the other hand, the thickness of the layer deposited by sputtering with no bias or smaller bias may be preferably the same or larger than that obtained by the radio frequency sputtering.
The term “layers” here does not mean layers of different materials but layers having different concentrations of the sputtering gas obtained by varying the magnitude of the bias.
Advantages of the Invention
The present invention thus makes it possible to produce a wear-resistant protective film which has little possibility of cracking due to internal stresses or step coverage. Use of smaller bias in place of no bias increases the adhesion to the thermal head. This can be explained as follows. The layer formed under no bias and the layer formed under a larger bias create tensile stress and compression stress, respectively, and thus their combination produces a large shearing stress between them. On the other hand, the layer formed under a smaller bias and the layer formed under a larger bias create both compression stresses, respectively, and thus their combination produces a small shearing stress between them. Variation of bias voltage during sputtering is not suggested in the above-cited publication. From the foregoing, a protective film having no crack owing to the internal stress nor crack due to the poor step coverage is provided.
Another advantage of the present invention is the productivity of the protective film since the film having different concentrations of sputtering gas in the direction of the film thickness can be formed by using a single apparatus with a single target to be sputtered.